Mitsuru Ohata

Damage concept for evaluating ductile cracking of steel structure subjected to large-scale cyclic straining

Abstract Evaluation of ductile crack initiation in steel welded structures subjected to seismic loading is crucial for structural design or safety assessment to prevent brittle fracture induced by ductile cracking. Observation of ductile crack initiation behavior of round-bar specimens with/without circumferential notches tested in single tension revealed that the main controlling factor for ductile cracking in the employed two-phase steel is not growth of voids induced by large inclusions, but nucleation of micro-voids in a soft phase (Ferrite phase) near the Ferrite–Pearlite interface after large-scale plastic straining. The material damage concept under reverse loading, which correlates the material damage for micro-void nucleation to macro-scale mechanical parameters, was proposed in consideration of two aspects of the Bauschinger effect: (a) a mechanical aspect which influences deformation and stress/strain behaviors in steel structures, (b) a material damage aspect caused by dislocation behavior. A new criterion for ductile cracking of structural members under cyclic loading was proposed on the basis of the proposed effective damage concept and ‘two-parameter criterion,’ which can be applied to the steel structures under increasing load in a single direction. The validity of the advanced two-parameter criterion was verified by subjecting round-bar specimens to cyclic loading tests along the axial direction and cross-shaped specimens to cyclic 3-point bending tests. Consequently, the advanced two-parameter ductile cracking criterion was found to be a transferable criterion for evaluation of critical loading cycle of structural members from small-scale tensile test results.

Damage Model for Predicting the Effect of Steel Properties on Ductile Crack Growth Resistance

This study pays attention to reveal material properties that control a resistance curve for ductile crack growth (CTOD-R curve) on the basis of the mechanism for ductile crack growth, so that the R-curve could be numerically predicted only from those properties. Crack growth tests using 3-point bend specimens with a fatigue pre-crack are conducted for two steels that have different ductile crack growth resistance, whereas both steels have the same mechanical properties in terms of strength and work hardening. Observation of crack growth behaviors provides that different mechanisms between ductile crack initiation from fatigue pre-crack and subsequent growth process can be applied. It is shown that two types of ductile properties of steel associated with ductile damage can mainly influence CTOD-R curve; one is a resistance of ductile crack initiation estimated with critical local strain for ductile cracking from a surface of notch root, and the other one is a stress triaxiality dependent ductility obtained with circumferentially notched round-bar specimens. The damage model for numerically simulating the R-curve is proposed taking the above two ductile properties into account, where the ductile crack initiation from crack-tip is in accordance with local strain criterion, and the subsequent crack growth triaxiality dependent damage criterion. The proposed model accurately predicts the measured different R-curves between two steels used that have the same ‘strength properties’, and also the stress triaxiality dependence of R-curve.

Fracture Toughness Evaluation of Laser Beam-Welded Joints of 780 MPa-Strength Class Steel

For fracture assessment of steel structures with laser beam-welded joints, it is significantly important to evaluate the fracture toughness of the weld metal. However, the fracture toughness of weld metal is often impossible to measure by standard Charpy impact testing in the brittle-to-ductile transition or upper-shelf temperature region, because of fracture path deviation (FPD) from the weld metal due to narrow weld bead and a high degree of overmatching in strength. In this work, evaluated fracture toughness of laser beam-welded joints of 780 MPa-strength class (HT780) steel is examined by Charpy impact testing and CTOD fracture toughness testing. The improved Charpy impact tests, using the specimen with three parallel welds or the side-groove specimen, are conducted in order to prevent FPD. The applicability and significance of the improved impact test methods to the laser beam-welded joints of high-strength steel are investigated by means of Weibull stress analysis. The Weibull stress analysis indicates that the 3-weld method could not necessarily prevent FPD in laser beam-welded joints of HT780 steel, since side beads are not as hard as main weld beads, due to reheating at the main weld. On the other hand, a side-groove specimen is effective for avoiding FPD, but intrinsic VE of the weld metal could not necessarily be obtained by using this specimen.

Damage Mechanism for Controlling Ductile Cracking of Structural Steel with Heterogeneous Microstructure

This study is focused on the experimental clarification of ductile cracking controlling mechanism in structural steel with heterogeneous microstructures. The structural steel with Ferrite-Pearlite two-phases with different strength in Vickers hardness was used. The ductile cracking tests were conducted under single tensile loading for round-bar specimens with/without circumferential notch in order to compare the ductile cracking behaviors under different triaxial stress state during loading. The observation of voids nucleation and growth behaviors up to ductile crack nucleation in the middle of the minimum cross-section of the specimens indicated that the damage for ductile cracking would be localization of plastic strain up to voids nucleation at the Ferrite-Pearlite boundary. This behavior was applied irrespective of the triaxial stress state during loading. Consequently, the main controlling factor for ductile cracking in the employed two-phase steel was found to be not growth of voids induced by larger inclusions, but nucleation of voids in softer phase (Ferrite phase) near Ferrite-Pearlite boudnary after large-scale plastic straining.

THE INFLUENCE OF SOFT ROOT ON FRACTURE BEHAVIORS OF HIGH-STRENGTH, LOW-ALLOYED (HSLA) STEEL WELDMENTS

The presence of different microstructures along the precrack fatigue front has an important effect on the critical crack tip opening displacement (CTOD). This value is the relevant parameter for safe service of a welded structure. For a specimen with a through-thickness notch partly in the weld metal, partly in the heat-affected zone, and partly in the base material (i.e., using the composite-notched specimen), fracture behavior strongly depends on the proportion of ductile base material, size, and distribution of the mismatching factor along the vicinity of the crack front.

Effect of Close-Packed Plane Boundaries in a Bain Zone on the Crack Path in Simulated Coarse-Grained HAZ of Bainitic Steel

Global effect of Bain-zone boundaries and the local effect of close-packed plane (CP) boundaries on the path of secondary cleavage cracks (observed on a fractured V-notch Charpy specimen) were visualized and discussed in simulated coarse-grained HAZ of bainitic steel. Microstructural unit map (Bain-zone map and CP map) was obtained by electron backscatter diffraction method for a prior austenite grain of a few hundred micrometers. Furthermore, a correlation between CP boundaries and sites that favored formation of the blocky martensite–austenite (M–A) constituent was confirmed. It was clarified that two crack deviation/local changing factors (CP boundaries and blocky M–A) are paired.

Significance of Strength Mismatch in Fracture Performance Evaluation of Welded Joints — Characterization of Mismatch Effect by the Weibull Stress Criterion

The effect of strength mismatch in welds on the fracture driving force of the heat-affected zone (HAZ) is investigated by 3-dimensional FE-analysis. The Weibull stress is used for the evaluation of the fracture driving force. The CTOD ratio, γ = δMismatch /δMatch, is proposed to quantify the mismatch effect, where δMismatch and δMatch are CTODs of the mismatched joint and the matched joint, respectively, at the same level of the Weibull stress. The CTOD ratio γ is decreased by strength overmatching, which is due to the constraint effect of the overmatch WM on the stress fields in the HAZ. The equivalent CTOD ratio, β = δ/δWP, is employed to correct the constraint loss in the structural component, where δ and δWP are CTODs of the fracture toughness specimen and the wide plate component, respectively, at the same level of the Weibull stress. The analysis using β and γ indicates that strength overmatching leads to a severe requirement of the HAZ toughness, which cannot be directly derived from the CTOD — remote strain relationship for the wide plate joint. By contrast, the toughness requirement of the WM is relaxed by strength overmatching, as a result of the shielding effect of the overmatch WM.

Evaluation method for Charpy impact toughness of laser welds based on lateral contraction analysis

An evaluation of Charpy impact toughness of the weld metal for laser beam welds is often impossible due to occurrence of fracture path deviation (FPD) from the weld metal into base steel. Therefore, the improved toughness evaluation method using side-grooved specimen instead of normal specimen has been suggested for preventing FPD. However, for a pre-qualification of the toughness of laser welds, the toughness value obtained with side-grooved specimen should be corrected to the toughness value of normal Charpy specimen. In this study, the method for correction of side-grooved Charpy energy for laser welds to normal Charpy impact toughness suggested by Hagihara et al. (Q J Jpn Weld Soc 25: 165–172, 2007) is advanced by means of FE analysis with focus on the deformation behaviors associated with strength mismatch. Furthermore, the distinctive strength mismatch of laser welds on lateral contraction of Charpy specimen has been analyzed by means of the Weibull stress approach. The applicability of the advanced engineering method is demonstrated by comparing experimental result, whereas the further verification should be accumulated.

Fracture Mechanics Assessment of Beam-to-Column Joints Subjected to Cyclic and Dynamic Loading

With a lesson learned from the Kobe Great Earthquake, an engineering method, WES 2808, has been developed in Japan for evaluating the structural integrity under the seismic condition. This paper presents a method for fracture mechanics assessment of beam-to-column joints subjected to cyclic and dynamic loading on the basis of WES 2808. WES 2808 includes two key ideas. One is a reference temperature concept for fracture toughness evaluation. The material fracture toughness under the seismic condition is replaced by the static fracture toughness without pre-strain at a reference temperature of T−ΔTpd, where T and ΔTpd are the service temperature and the temperature shift of the fracture toughness by pre-strain and dynamic loading, respectively. ΔTpd is given as a function of the flow stress elevation in the seismic condition. Another idea is a correction of constraint loss in structural components. A large amount of plastic deformation causes a loss of plastic constraint in structural components. In order to account for the constraint loss in the fracture assessment of components, the equivalent CTOD ratio β = δ3p / δStruc. is implemented, where δ3p and δStruc. are CTODs of the fracture toughness specimen and the structural component, respectively, at the same level of the Weibull stress.

Effect of Strength Mismatch on Ductile Crack Initiation Behavior from Notch Root under Static Loading

It has been well known that ductile fractures of steels are accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using two-parameter criterion based on equivalent plastic strain and stress triaxiality. It has been demonstrated by authors using round-bar specimens with circumferential notch in single tension that the critical strain to initiate ductile crack from specimen center depends considerably on stress triaxiality, but surface cracking of notch root is in accordance with constant strain condition. This study fundamentally clarifies the effect of strength mismatch, which can elevate plastic constraint due to heterogeneous plastic straining under static loading, on critical conditions for ductile cracking from the pre-notch root. In order to evaluate the stress/strain state in the pre-notch root of specimens, a thermal elastic-plastic finite element (FE) analysis has been carried out.